Accurate knowledge of the refractive index of materials is very important and useful in many fields of technology. For example, the refractive index can be used to determine substance composition for chemical analysis; it is used in the design of optical systems that include refracting elements to optimize imaging quality; and it may also be used for material identification and characterization, among others. For optical materials, the refractive index is generally defined as the ratio of the speed of light in vacuum to that in the material itself. Although refractive index data is readily available for almost any known transparent or crystalline material, it is usually given for only one or a few wavelengths of the electromagnetic spectrum. In addition, the refractive index of a given material may vary in the presence of externally applied magnetic or electric fields, or the temperature of the material, among others. Accordingly, there is a need to more accurately measure the index of refraction of materials.
A variety of instruments and techniques for determining the refractive index of materials are currently available. Among those, interferometry is a well known technique for measuring the refractive index of a substance. The measurement of refractive index using a two-beam interferometer may be accomplished by placing a sample of known thickness in one of the beams and determining the change in the order of the interference fringes. Representative examples of conventional interferometry are discussed below.
Kim et al., U.S. Pat. No. 6,545,763 (hereafter “Kim”), discloses a method for measuring a thickness profile and a refractive index using white-light scanning interferometry. Kim discloses using white light to perform low coherence interferometry to measure the phase graph (phase delay profile along the z-direction of a sample); extracting a mathematical phase graph through modeling of a measurement object; and estimating the sample thickness and refractive index by minimizing the difference between measured and simulated phase graphs. However, because the refractive index is a function of the wavelength of light, using white light as the light source does not provide an absolute and accurate measurement. In addition, the phase profile measured along the z-direction of the sample depends on both the physical movement and the sample refractive index. Therefore, the accuracy of the index measurement depends on the accuracy of the z-movement measurement.
Bornhop et al., U.S. Pat. No. 7,130,060 (hereafter “Bornhop”), discloses a technique for refractive index determination by micro interferometric reflection detection. Bornhop discloses placing a sample of interest (liquid) into a capillary tube with a known physical configuration. The known refractive index of the capillary tube is used as a reference for the liquid sample. Therefore, a precise knowledge of the refractive index of the capillary is required for determining the refractive index of the liquid. In other words, the accuracy of the liquid index measurements is limited by the accuracy of the knowledge of the refractive index of the capillary tube which acts like an “index reference.” This greatly limits the accuracy of this technique.
Kun-I Yuan, U.S. Pat. No. 7,663,765 (hereafter “Yuan”), discloses a refractive-index measurement system that measures a “change” in refractive index of a lens placed inside a container. The container accommodates the lens therein and is filled with a medium having a refractive index substantially the same as a theoretical refractive index of the lens. Interference fringes of a first light beam transmitted through different points (a first point and a second point) of the sample and a second light beam not transmitted through the sample are counted. The change of refractive index of the lens is obtained by comparing the value of refractive index of the lens at the second point with the theoretical refractive index of the lens at the first point. Yuan does not disclose how to measure the absolute refractive index, but only a relative change in refractive index. Moreover, according to Yuan, the accuracy of the measurement relies on the assumption that the medium in which lens is placed has a refractive index substantially the same as a theoretical refractive index of the lens.
What is needed, therefore, is a technique for reliably and accurately measuring the refractive index of a sample in a simple and efficient manner.